1. Field of the Invention
The present invention relates generally to measuring the contents of a vessel and more specifically to such a technique and an apparatus which enables the content of a vessel to be determined without the need for immersing probes and the like into the fluid or material which is stored in the vessel.
2. Description of the Prior Art
FIGS. 1 to 5 show a prior art vessel content measuring apparatus disclosed in JP-A-63-108231 which was provisionally published on May 13, 1985.
As shown schematically in FIG. 1 the vessel or tank 1 which in this instance is an automotive fuel tank (merely by way of example) is partially filled with a liquid fuel in a manner to define an air space therein. A fuel line 1a leads from the bottom of the tank and leads to an engine not shown. The upper portion of the fuel tank is formed with a filler neck 1b which is sealingly closed by a cap 1c. A conduit 2 leads from the neck to a fuel volume sensing arrangement generally denoted by the numeral 3.
This arrangement comprises a first chamber 4 which will be referred to as a compensation chamber which communicates with the conduit 2 via an ON/OFF type valve 6. A sub-chamber 8 is fluidly communicated with the conduit 2 at a location between the fuel tank 1 and the valve 6.
A pressure sensor arrangement 10 of the nature shown in FIG. 2 is disposed in the compensation tank 4. This sensor 10 includes a diaphragm 10a which divides the sensor housing into a first chamber pressure sensing chamber 10b which is in fluid communication with the compensation chamber 4 and a second standard or reference pressure chamber 10c which is in fluid communication with the sub-chamber 8. A strain gauge type pressure sensing element 10e is mounted on the diaphragm 10a and arranged to output a signal indicative of the amount flexure the diaphragm undergoes. This signal is supplied to a circuit arrangement the nature of which will be disclosed in detail later.
A cylinder 12 in which a reciprocal piston 12a is disposed is fluidly communicated with the compensation chamber. The piston 12a is provided with a special seal 12b arrangement which ensures a hermetic seal is formed between the outer periphery of the piston 12a and the bore of the cylinder 12.
The piston 12a is connected by way of connecting rod 13 with a crank 14. The crank 14 is in drive connection with an electric motor 16 by way of a reduction gear 17 and further provided with a suitable marker 14a which cooperates with a light sensor 18 which is located immediately adjacent the periphery of the crank 14.
The light sensor 18 is arranged to detect the passing of the marker 14a and output a signal to a motor control circuit 20. This circuit 20 is arranged to energize the motor 16 in a manner which maintains the angular rotational speed of the crank 14 constant at a predetermined level.
The pressure signal generated by the pressure sensor 10 is supplied firstly to a band pass filter 22 which is set in accordance with the predetermined angular rotational speed of the crank in order to screen out noise and the effects of temperature related drift. The output of the band pass filter 22 is next supplied to an amplitude detection circuit 23 wherein the peak amplitude value is detected. An APU circuit 24 which includes in this instance a microprocessor (CPU, ROM etc) is arranged to receive the output from the amplitude detection circuit 23 and to derive the volume of the liquid which is contained in the fuel tank 1, issue a suitable signal to a display unit 25 which indicates the volume; and output control signals to the valve 6 and the motor control circuit 20.
Before proceeding with an explanation of the operation of the above arrangement, it is deemed appropriate to turn to FIGS. 3 and 4 and to consider the concept on which the above measuring system is based.
Under the conditions shown in shown in FIG. 3 wherein the piston 12a is at BDC and the valve 6 is open, and further given that:
VT denotes the total volume of the vessel 1; PA1 V1 denotes the volume of the correction chamber 4; PA1 vo denotes the maximum volume change which can occur in said cylinder 12 (&lt;&lt;V1,V2); PA1 V2 denotes the volume of the vessel 1 not occupied by the liquid medium; PA1 VL denotes the volume of the liquid (solid or powder etc.) contained in the vessel 1 (main tank) and PA1 Po denotes the pressure within the vessel 1: PA1 n denotes the number of mols contained in the cylinder 12, compensation chamber 4 and the portion of the vessel or tank 1 which is not filled with liquid; PA1 R is a gas constant; PA1 To is the absolute temperature and .gamma. is the ratio of isopiestic specific heat and isovolumic specific heat. PA1 In this situation, if the piston 12a is stroked adiabatically, as shown in FIG. 3, vo=0 and at the same time the pressure in the tank is increased by .DELTA.Po.
then, according to Poisson's Law it can be shown that: EQU Po(V2+vo+V1).gamma.=nRTo (1)
wherein:
Therefore: EQU (Po+.DELTA.Po) (V2+v1).gamma.=nRTo (2)
Accordingly, from equations (1) and (2) it can be shown that: EQU Po (V2+vo+V1).gamma.=(Po+.DELTA.Po) (V2+V1).gamma. (3)
Via approximation it is possible to express the above as: ##EQU1##
By rearrangement, the unfilled volume V2 of the tank can be shown to be: ##EQU2##
In the case the valve 6 is closed and the communication between the compensation chamber 4 and the interior of the tank 1 is cut-off and the situation shown in FIG. 3 becomes such that: EQU Po (vo+V1).gamma.=nRTo (6)
while in the case of the situation shown in FIG. 4 (Viz., under the conditions wherein valve 6 is open) EQU (Po+.DELTA.Po') V1.gamma.=nRTo (7)
Thus, similar to the above equations 6 and 7 can be equated to give: EQU Po (vo+V1).gamma.=(Po+.DELTA.Po') V1.gamma. (8)
and via approximation ##EQU3##
By rearrangement ##EQU4##
Given that vo and V1 are known, .DELTA.Po and .gamma.Po can be readily derived.
Therefore using equation 5 it is possible to monitor the output of the pressure sensor 10 and derive V2. VL (the volume of liquid in the tank) can then be determined by subtracting V2 from the total tank volume VT (viz., Vt -V2)
The operation of the arrangement shown in FIG. 1 will now be discussed with reference to the flow chart shown in FIG. 5.
The first step 1001 of the routine depicted in this flow chart is such as initialize the system. A predetermined period after the initialization, a command which opens the valve 6 is issued in step 1002. This command induces the issuance of a signal which is supplied to the valve 6 and which induces the same to open. Following the opening of the valve 6, piston 12a is permitted to reciprocate a plurality of times during which a plurality .DELTA.Po' readings are taken and averaged. Using this data and equation 10, the value of .gamma.Po is derived. That is to say, the ROM of the microprocessor contains pre-recorded data pertaining to the volume V1 of the compensation chamber and the maximum volume change vo of the cylinder, whereby, when in possession of the average .DELTA.Po' data, it is possible to calculate .gamma.Po using the equation: ##EQU5##
Following this, a command is issued to close the valve 6 (step 1004) and in step 1005 the volume V2 of the tank which is filled with gas (air), is calculated. Following this, at step 1006 the value VT which was read of ROM along with a number of other pre-recorded data for the purposes of the V2 calculation, is used in step in order to determine the amount of liquid in the tank (i.e. VT-V2=VL). At step 1007 signals which induce the display of the "level" of the fuel in the tank 1 are issued to the display and the program recycles to step 1002.
It should be noted that this routine includes a non-illustrated cancellation step which is arranged between steps 1005 and 1006 which is responsive to abnormally large changes in the gaseous volume V2.
More specifically, when the power is supplied to the circuit arrangement, the output of the light sensor 18 is supplied to the motor control circuit 20 and the crank 16 is rotated until it is aligned with the marker 14a. Following this, a signal is issued to the valve 6 which induces the same to assume a closed condition. At the same time a signal is supplied to the motor control circuit 20 which induces a predetermined number of motor rotations at a predetermined rotational speed. This induces the crank 14 to be rotated and the piston 12a to be reciprocated in the cylinder 12 in a manner wherein the volume of air V1 the cylinder is totally displaced into the compensation chamber 4 and then re-induced.
This causes the pressure prevailing in the compensation tank 4 to rise and fall. In response to this the pressure prevailing in the pressure sensing chamber 10b of the pressure sensor 10 is compared with that prevailing in the reference chamber 8 and the resulting flexure of the diaphragm 10a is translated into an electrical signal by the strain gauge 10e. This signal is supplied by way of the band pass filter 22 to the amplitude detection circuit 23 wherein in the peak value of the signal is determined. The value of .gamma.Po is the derived by the APU 24 and recorded in a register of the CPU included therein.
Following this, the APU 24 issues a signal which cancels the valve closure and permits the same to open. The motor 16 is then induced to rotate for a second predetermined number of revolutions which is greater than the predetermined number of rotations induced during the operation wherein the data for the derivation of .DELTA.Po was collected. During this second set of rotations the data required for the solving of equation 5 is collected, the requisite data read of ROM, and the volume V2 of the unfilled portion V2 of the tank 1 is derived using the coefficient .DELTA.Po.
The amount of liquid VL in the tank is derived by subtraction as indicated above.
However, this arrangement has suffered from the drawbacks that in order to derive the coefficient .DELTA.Po, it is necessary to provide the compensation tank 4 and and valve 10 and as such it is not possible to render the arrangement sufficiently compact as to enable the ready deployment in vehicles and the like, wherein space is at a premium.
Further, as the valve 6 defines a passage structure which has a limited cross sectional area (when open), a flow resistance tends to be produced which interferes with the transmission of pressure between the tank 1 and the compensation chamber 4 and thus introduces an error into the measurement. To solve this problem it has been considered to increase the size of the valve 6. However, this increases the bulk and cost of the arrangement.